The present invention relates to equipment for use in an optical recording system, and it relates especially, but not exclusively, to such equipment for use in an optical tape recording system.
In a helical scan optical tape recorder with optical components external to the rotating drum, around which the tape is caused to pass for transducing purposes, it is necessary to overcome the problems caused by rotation of the optical image as directed to the tape. This compensation can be achieved by use of a second rotating element.
However, for a single track system, the situation can be simplified by matching the axis of the external optical system to the axis of rotation of the drum. If this is done then the focussed spot image merely rotates about its own axis of symmetry, thus avoiding the need for de-rotation of the image. This is particularly so if the spot itself is truly circularly symmetrical. This invocation of symmetry provides a single channel optical tape recorder which does not require additional optical components in order to effectively "de-rotate" the image as directed on to the tape.
The use of axial symmetry for a single channel tape recorder is applicable regardless of changes in the focus of the system, and so an external focal tracking mechanism (for example similar to one used in compact disc technology) is viable.
Unlike focal tracking, however, the lateral (i.e. transverse to the recorded tracks) tracking of data does not preserve axial symmetry. For an external tracking servo mechanism to operate, it needs to generate a circular tracking motion. This can be achieved with an x/y scanner fed with out-of-phase sine waves. However the rotating nature of the spot image makes the acquisition of a suitable tracking error signal extremely difficult. Therefore in a single axis system the tracking detection, at least, is placed within the drum.